Various mobile or portable electronic devices may have reduced power consumption by operating some of the systems within these devices at low voltages (e.g., 3.0 volts, 1.5 volts, etc.). Such electronic devices often use direct current to direct current converters (“dc to dc converters” or “dc-dc converters”) to “step down” voltages available from their power supplies to the lower voltages used by these systems.
Typical dc-dc converters include switched capacitor dc-dc converters, which may contain one or more switches controlling one or more energy storage elements (e.g., “flying capacitors,” etc.). The switches determine when the energy storage elements charge and discharge to supply power to the load. The energy storage elements may charge from a regulated current source and may discharge at least in part to a “buffer” or output capacitor coupled in parallel with the load.
The switches used in switched capacitor dc-dc converters, particularly those implemented in sub-micrometer technologies, can suffer from a condition referred to as hot carrier stress (HCS), which may degrade the performance of the switch over time. High-energy carriers, also called hot carriers, can be generated in a switch as a result of high energy electric fields generated around the devices during operation. The presence of such hot carriers triggers numerous physical damage processes, referred to as HCS, which can change the switch characteristics over the life-time of the device. Long-term operation can cause malfunction of circuits using switches, such as dc-dc converters.
For example, high energy electric fields may accelerate local carriers in switch materials (e.g., semiconductor materials) to effective temperatures well above the lattice temperature. The hot carriers can transfer kinetic energy to the lattice that may break bonds at the Si/SiO2 interface. Also, as a result of the high energy electric fields, carriers can be injected into the SiO2 layer, and can become trapped there.
Incidents such as bonds broken or carriers becoming trapped may create an undesired charge in the SiO2 layer and/or cause an interface trap that can reduce channel carrier mobility and increase the threshold voltage of the device. Related problems caused by HCS in various devices can include substrate current due to hot carrier flow into the bulk, device degradation from hot carrier flow into the gate, decreased drain current, decreased transconductance, and degradation of the sub-threshold slope.
Some solutions have been offered to reduce the effects of HCS. Many, if not all of the solutions offered reduce hot carrier degradation at the cost of an increase in the on-resistance/impedance (RON) of the switch or an increase in the area of the switch (to achieve a same RON value). The RON of a switch and the area of the switch are both important performance parameters in a switched capacitor dc-dc converter, particularly when implemented in sub-micrometer technologies. Generally, the lower the RON, the better the performance, and the smaller the area, the better the device meets implementation or design standards.